The present invention relates to a switched-capacitor filter having, at its input stage, a prefilter.
Stimulated by progress in MOS (metal oxide semiconductor) technology, considerable interest has recently arisen in the possibility of a filter realized by means of switched-capacitors, i.e. a switched-capacitor filter (hereinafter often referred to by the abbreviation "SCF"). The major advantages of the SCF reside in the facts that only capacitors, operational amplifiers, and switches are needed, that nearly perfect switches can easily be built, and, especially, that all resonant frequencies are determined exclusively by capacitance ratios. Therefore, the SCF is very useful in various kinds of electronic processing systems, for example, PCM (pulse code modulation) communication systems. In a PCM communication system, as will be explained hereinafter, a low-pass filter is one of the important electronic processing members. The low-pass filter operates in such a manner as to cut off signals having a frequency higher than 4 kHz and, accordingly, only voice signals can pass therethrough. In a PCM communications system, it is preferable to build the low-pass filter by means of the SCF.
Although the SCF has the above mentioned advantages, it also has the disadvantage that, generally, the SCF is driven through a sampling operation. The sampling frequency for achieving the sampling operation is, for example, 128 kHz. In this case, since the SCF processes successive sampled voice data synchronously with said sampling frequency, a so-called alias signal is generated. If the SCF receives a signal having the frequency of, for example, (128+1) kHz, an undesired signal of 1 kHz appears as the alias signal in a base band signal. This signal of 1 kHz is not a true voice signal and, accordingly, it should be eliminated. Consequently, a prefilter is added to the input stage of the SCF, in order to suppress said signal of (128+1) kHz. Generally, the prefilter should suppress signals of [128n.+-.(0 through 4)] kHz (n=1, 2, 3 . . . ) and their respective neighbouring signals, where the frequency of (0 through 4) kHz corresponds to the base band frequency representing the voice signal. Thus, the prefilter is also called an anti-alias filter.
In the prior art, the prefilter is built as a second order RC active filter which is comprised of an operational amplifier, capacitors and resistors. However, the second order RC active filter is not preferable as a prefilter from an economical point of view. This is because the active filter includes the operational amplifier which requires energizing power. That is, the power consumption is relatively large. This fact corresponds to the first diseconomy. Further, the single chip IC (integrated circuit) for fabricating the SCF is partially occupied by the operational amplifier of the RC active filter. That is, a high density IC can not be obtained due to the presence of the operational amplifier. This fact corresponds to the second diseconomy. It should be understood that, in each PCM communication system, since there are several thousand communication channels, several thousand operational amplifiers must be incorporated in the ICs. Accordingly, the power consumption becomes extremely large, and, moreover, a high density IC cannot be expected.